It is known that, in the natural state, wood or wood fibers which are in contact with a humid atmosphere tend to be water-logged, going as far as absorbing 100% of their weight with water. Such absorption of water is accompanied by a swelling, characteristic of a reduction in the qualities of cohesion of the material which, in certain cases, may go as far as an advanced disintegration of said material. This is why it is usual to effect, before any wood-fashioning operation, a step of drying which, by eliminating the water therefrom, improves its dimensional stability.
Although the step of drying makes it possible to eliminate the water from the wood, it does not, on the contrary, modify the hydrophilic nature thereof, so that it is again capable of reabsorbing the water eliminated during drying, when it is again located in a humid atmosphere.
In order to reduce the hydrophilic nature of natural wood, and thus to give it a long-lasting dimensional stability, different high-temperature heat-treatment techniques have been proposed.
Among these techniques, it has been proposed to subject the natural wood to different steps of treatment including in particular a drying in open circuit followed by heating and maintaining at a temperature included between about 220.degree. C. and 300.degree. C. for a determined period. Such a technique of treatment, called controlled thermal treatment (curing), makes it possible to give the wood both a hydrophobic nature and an excellent dimensional stability.
It has also been proposed, particularly in order to improve the mechanical properties of the wood, to call upon techniques of treatment which consist in impregnating the wood with a monomer then, such impregnation having been effected, in polymerizing it in situ, employing different techniques to that end, such as in particular the action of a gamma radiation or the action of heat.
The methods of treatment used in the prior state of the art consist in disposing the wood to be treated in a chamber in which a vacuum is created, in filling said chamber with the impregnating monomer, and in applying a high pressure in this chamber so as to cause the monomer to penetrate in the wood. All that remains is to polymerize the monomer.
Although these techniques prove to be satisfactory from the standpoint of improving the mechanical properties of the treated wood, and particularly from that of the hardness, they are much less so from that of the stability by volume of the wood in a humid atmosphere. It has thus been noted that the woods thus impregnated presented the particularity, after a certain time of use, of losing their adherence with the polymer with which they were impregnated, which was translated by this wood swelling.